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Handling Errors

Sending custom error data in the graphql response

Returning errors

All resolvers simply return an error to send it to the user. Its assumed that any error message returned here is safe for users, if certain messages arent safe, customize the error presenter.

Multiple errors

To return multiple errors you can call the graphql.Error functions like so:

package foo

import (
	"context"
	
	"github.com/vektah/gqlparser/gqlerror"
	"github.com/99designs/gqlgen/graphql"
)

func (r Query) DoThings(ctx context.Context) (bool, error) {
	// Print a formatted string
	graphql.AddErrorf(ctx, "Error %d", 1)

	// Pass an existing error out
	graphql.AddError(ctx, gqlerror.Errorf("zzzzzt"))

	// Or fully customize the error
	graphql.AddError(ctx, &gqlerror.Error{
		Message: "A descriptive error message",
		Extensions: map[string]interface{}{
			"code": "10-4",
		},
	})

	// And you can still return an error if you need
	return false, gqlerror.Errorf("BOOM! Headshot")
}

They will be returned in the same order in the response, eg:

{
  "data": {
    "todo": null
  },
  "errors": [
    { "message": "Error 1", "path": [ "todo" ] },
    { "message": "zzzzzt", "path": [ "todo" ] },
    { "message": "A descriptive error message", "path": [ "todo" ], "extensions": { "code": "10-4" } },
    { "message": "BOOM! Headshot", "path": [ "todo" ] }
  ]
}

Hooks

The error presenter

All errors returned by resolvers, or from validation pass through a hook before being displayed to the user. This hook gives you the ability to customize errors however makes sense in your app.

The default error presenter will capture the resolver path and use the Error() message in the response. It will also call an Extensions() method if one is present to return graphql extensions.

You change this when creating the handler:

server := handler.GraphQL(MakeExecutableSchema(resolvers),
	handler.ErrorPresenter(
		func(ctx context.Context, e error) *gqlerror.Error {
			// any special logic you want to do here. Must specify path for correct null bubbling behaviour.
			if myError, ok := e.(MyError) ; ok {
				return gqlerror.ErrorPathf(graphql.GetResolverContext(ctx).Path(), "Eeek!")
			}

			return graphql.DefaultErrorPresenter(ctx, e)
		}
	),
)

This function will be called with the the same resolver context that threw generated it, so you can extract the current resolver path and whatever other state you might want to notify the client about.

The panic handler

There is also a panic handler, called whenever a panic happens to gracefully return a message to the user before stopping parsing. This is a good spot to notify your bug tracker and send a custom message to the user. Any errors returned from here will also go through the error presenter.

You change this when creating the handler:

server := handler.GraphQL(MakeExecutableSchema(resolvers),
	handler.RecoverFunc(func(ctx context.Context, err interface{}) error {
		// notify bug tracker...

		return fmt.Errorf("Internal server error!")
	}
}